Notch Effect on Strain-Controlled Low Cycle Fatigue Strength of 80kg/mm2 High Tensile Steel

Abstract

This paper deals with the results of the fatigue test on hourglass type and round notched specimens of 80 kg/mm2 high strength steel. Strain controlled low cycle fatigue tests were carried out to investigate the notch effect on the fatigue strength reduction factor and the life ratio in the low cycle range. Two kinds of hourglass type and four kinds of round notched ones were tested. The theoretical stress concentration factor were varied in five conditions.The following conclusions were obtained : (1) The natural fracture strain in static tension, ef, is decreased as the theoretical stress concentration factor, Kt, increased. The degree of decrease of ef is large in the range of 1 to 1. 6 of Kt. (2) The relation between total true strain amplitude and number of cycles to the initiation of visible crack on the surface of specimen, Nc, was obtained as eta=CeNc-ke=CpNp-kp. (3) The values of C and k can be determined in the Kt-eta and Kt-C curves in the range of 1 to 4 of Kt value. For the condition that Kt is equal to 1. 00 the eta-Nc relation was obtained as eta=0. 36 Nc-0.50+0. 0054 Nc-0.087.(4) The relation between propagation life and failure life is represented as, Nf-Nc=0. 607 Nf0.89.(5) The fatigue strength reduction factor, Kf, e, was defined as the ratio of total true strain amplitude of smooth specimen (K=1.00) to that of notched specimen. Kf, e is always higher than K in the low cycle range. (6) The life ratio was defined as the ratio of failure life of smooth specimen (K=1. 00) to that of notched specimen. Life ratio increase logarithmicly with increasing of Kt value.